This invention relates to an A-D converter by which an analog quantity having a non-linear characteristic such, for example, as an output voltage with respect to a temperature of a thermocouple is converted into a digital quantity.
A dual slope system is usually employed for converting a voltage to be measured into a digital value. In the dual slope system, the voltage to be measured is subjected to first integration by an integrator for a certain period of time and a reference voltage of the opposite polarity from the voltage to be measured is integrated until the integrated value obtained in the integrator in the period of the first integration returns to a predetermined value, for example, to zero. In the period of the second integration, a gate circuit is opened, through which clock pulses are supplied to a counter and, by counting the number of clock pulses fed to the counter in the period of the second integration, a digital value corresponding to the voltage to be measured is obtained.
In the case of an A-D conversion of the output voltage of a thermocouple by the employment of the dual slope system for the purpose of a digital indication of the temperature of the thermocouple, since the relation between the temperature and output voltage of the thermocouple has a non-linear characteristic, it is necessary that an A-D converter of the dual slope system has a non-linear characteristic conforming to the above-said characteristic.
A method that has heretofore been employed to provide the A-D converter with such a non-linear characteristic is to change the integration time constant of the integrator each time the integrated voltage of the integrator varies a certain quantity in the period of the second integration. With this method, resistors are connected in series between the integrator and a reference power source for providing an integration time constant during the second integration and the resistance values of the resistors are successively changed as the second integration proceeds, by which the integration time constant of the integrator is changed to provide a non-linear relationship between an input voltage and its converted value.
With this method, however, the resistors for defining the time constant are required to be highly stable, which inevitably increases the cost of the A-D converter. Further, it is necessary to use resistors having desired resistance values, so that their adjustment is very troublesome. Moreover, in the case where thermocouples of different non-linear temperature vs. output voltage characteristics are coupled to measuring terminals, the integration time constant is required to be changed in accordance with the individual non-linear characteristics, so that a certain combination of resistors must be provided. Accordingly, an increase in the number of kinds of thermocouples used increases the cost of the A-D converter.
An object of this invention is to provide a non-linear A-D converter which is free from a change resulting from aging, and hence excellent in stability.
Another object of this invention is to provide a non-linear A-D converter which can be obtained at a lost cost regardless of an increase in the number of kinds of transducers of different non-linear characteristics measurable with the converter.
Still another object of this invention is to provide a non-linear A-D converter which is inexpensive but highly accurate.